1. Field of the Invention
The present invention relates display devices, and more particularly, to a liquid crystal display (LCD) device and a driving method thereof for reducing or eliminating direct current image sticking and flicker.
2. Discussion of the Related Art
LCD devices display an image by controlling the light transmittance of liquid crystal cells in accordance with a video signal. FIG. 1 illustrates a liquid crystal cell of an active matrix type LCD device. In such an active matrix type LCD device, data voltages, which are supplied to liquid crystal cells Clc, are switched by thin film transistors (TFTs) formed in respective liquid crystal cells Clc under the active control of data to achieve an enhancement in the display quality of a moving image. In FIG. 1, the reference character “Cst” designates a storage capacitor to maintain the data voltage charged in the associated liquid crystal cell Clc, the reference character “DL” designates a data line to be supplied with the data voltage, and the reference character “GL” designates a gate line to be supplied with a scan voltage.
The LCD display device having the above described structure may be driven in accordance with an inversion scheme, in which polarity inversion not only occurs between neighboring liquid crystal cells, but also occurs at intervals of one frame, in order to reduce a DC offset of voltages applied to the cells and to reduce degradation of the liquid crystals. However, when any one of data voltages having opposite polarities is dominantly supplied for a prolonged period of time, image sticking may occur. Such image sticking is called “DC (direct current) image sticking” because it occurs when a liquid crystal cell is repeatedly charged with voltages having the same polarity. An example of such a case is the case in which data voltages are supplied to the LCD device in accordance with an interlace scheme. In accordance with the interlace scheme, data voltages are supplied to liquid crystal cells on odd horizontal lines in odd frame periods, while being supplied to liquid crystal cells on even horizontal lines in even frame periods.
FIG. 2 is a waveform diagram depicting an example in which data voltages are supplied to each liquid crystal cell Clc in accordance with the interlace scheme. In this example, it is assumed that the liquid crystal cell Clc supplied with the data voltages depicted in FIG. 2 is one of the liquid crystal cells arranged on one odd horizontal line.
Referring to FIG. 2, a positive voltage is supplied to the liquid crystal cell Clc in odd frame periods, and a negative voltage is supplied to the liquid crystal cell Clc in even frame periods. Since a data voltage having a high positive polarity level is supplied to liquid crystal cells Clc arranged on odd horizontal lines, only in odd frame periods, in accordance with the interlace scheme, the positive data voltage becomes dominant during the 4 frame periods in comparison to the negative voltage, as shown by the waveform in the box of FIG. 2. FIG. 3 is an image showing the experimental results of DC image sticking occurring due to interlace data. When an original image corresponding to the left image in FIG. 3 is supplied to an LCD panel for a certain period of time in accordance with the interlace scheme, the data voltage, which is varied in polarity at intervals of one frame, exhibits a considerable level difference between the odd frame and the even frame, as shown in FIG. 2. As a result, when a data voltage having an intermediate gray scale value, for example, a gray scale value of 127, is supplied to all liquid crystal cells Clc of the LCD panel, after the display of an original image such as the left image in FIG. 3, the pattern of the original image is dimly displayed, as shown by the right image in FIG. 3. That is, DC image sticking occurs.
Another example of DC image sticking may be the case in which an image is moved or scrolled at a certain speed. When an image is moved or scrolled at a certain speed, voltages of the same polarity may be repeatedly accumulated in each liquid crystal cell Clc in accordance with the correlation between the size of the scrolled figure and the scroll speed (moving speed). This example is illustrated in FIG. 4. FIG. 4 is an image showing the experimental results of DC image sticking occurring when an oblique line pattern or a character pattern is moved at a certain speed.
The moving image display quality of the LCD device may be degraded not only due to DC image sticking, but also due to flicker, namely, a periodic brightness difference visible to the naked eye of a viewer. Therefore, it is desirable to prevent the occurrence of DC image sticking and flicker, in order to enhance the display quality of the LCD device.